1. Field of the Invention
The present invention relates to a three-value power supply device adapted for use for the developing bias in an electrophotographic copying machine, a printer or a similar image forming apparatus, and an image forming apparatus utilizing said power supply device.
2. Related Background Art
Conventionally a sinusoidal or rectangular AC high voltage has been employed for the developing bias in the above-mentioned image forming apparatus. Also recently, rectangular waves of deviated duty ratios such as 4:6 or 3:7 are employed for improving the developing performance.
The sinusoidal wave or the rectangular wave of a duty ratio of 1:1 are generally obtained by elevating the voltage of a sinusoidal or square wave with a voltage elevating transformer. A DC high voltage to be superposed is generated by a DC-DC converter or the like and is supplied to an end of the secondary coil of said voltage elevating transformer.
The rectangular wave of the deviated duty ratio is generated by a method of modulating the primary and secondary sides of a high-frequency DC-DC converter with a low frequency.
On the other hand, for improving the developing performance, particularly for a high resolving power by preventing the toner scattering, there is required a three-value AC bias, or an AC high voltage having three levels of positive, negative and intermediate values. Also for improving the image quality, effective is faster upshift and downshift of the output wave form.
However, in comparison with the ordinary developing bias wave of a relatively low frequency in a range from several hundred Hz to 2 KHz, the positive and negative basic frequencies of the three-value bias are as high as 8 KHz, so that there is required a significant improvement in the upshift and downshift speeds.
Also the forming means for the intermediate level has been associated with the following drawbacks;
(1) The addition of a selector switch for the intermediate level involves a bidirectional switching circuit of a high speed and a high voltage resistance, and is practically unacceptable in consideration of the complication of the apparatus, and the increase in volume and cost, including the control circuit; PA1 (2) Also to realize the intermediate level by the discharge resistance only is not practical because of the significant power loss in said resistance; PA1 (3) A method of switching to a side only a timing of switching to the intermediate level and maintaining the intermediate level by a discharge resistance when the voltage comes close to said level has a drawback of very slow convergence to the intermediate level or a large overshoot for example because of fluctuation in the floating capacitance; PA1 (4) In a method of supplying the load with positive and negative outputs of DC high-voltage power source by selective switching with positive and negative high-voltage switches, if semiconductor switch devices are employed, such switch remains conductive by the feedback capacitance thereof even after the input signal is shut off at the timing of switching, thereby leading to a lowered upshift speed of output or a power loss. PA1 (1) A three-value power supply device having a high frequency-driven converter of a positive output; a high frequency-driven converter of a negative output; and high-speed switch for supplying or not supplying the output terminal with the positive or negative output of one of the high frequency-driven converters at desired timings of a frequency sufficiently lower than the high frequency; PA1 (2) A three-value power supply device provided with a high frequency-driven converter of a positive output, a high frequency-driven converter of a negative output, and a high-speed switch for supplying or not supplying the output terminal with the positive or negative output at desired timings of a frequency sufficiently lower than the high frequency. The device includes an amplitude detector for individually detecting the amplitudes of the positive and negative outputs appearing at the output terminal; a first comparator for comparing the amplitude of the positive output, detected by the amplitude detector, with a first reference value; a second comparator for comparing the amplitude of the negative output, detected by the amplitude detector, with a second reference value; a first high-frequency driver controlled by the output of the first comparator and adapted to drive the high frequency-driven converter of the positive output; and a second high-frequency driver controlled by the output of the second comparator and adapted to drive the high frequency-driven converter of the negative output; PA1 (3) A three-value power supply device having a high frequency-driven converter of a positive output; a high frequency-driven converter of a negative output; an output terminal receiving the outputs of the two converters; and a high-speed switch for supplying the primary side of converting transformers of the two converters with a high frequency signal, at desired timings of a frequency sufficiently lower than the high frequency, or short circuiting the primary side thereby supplying or not supplying the output terminal with the positive or negative output; PA1 (4) A three-value power supply device including a high voltage power source of a positive output; a high voltage power source of a negative output; first and second switching elements serially connected between the high voltage power sources of positive and negative outputs; a timing control device for selectively turning on and off the first and second switching elements; and an output terminal connected to the junction point between the first and second switching elements; PA1 (5) A three-value power supply device according to (1), wherein the timing control device is adapted to selectively turn on the first or second switching element only for a desired duration, at a timing when the output potential is switched from the positive or negative peak to the intermediate level; PA1 (6) A three-value power supply device including a high-voltage power source of a positive output; a high-voltage power source of a negative output; first and second switching elements serially connected between the high-voltage power sources of positive and negative outputs; an output terminal connected to the junction point between the first and second switching elements; an output detector for detecting the output of the output terminal; a comparator for comparing the output of the output detector with a reference signal; and a controller for selectively turning on and off the first and second switching elements; PA1 (7) A three-value power supply device having a high-voltage power source of a positive output; a high-voltage power source of a negative output; first and second switching elements serially connected between the high-voltage power sources of positive and negative outputs; an output terminal connected to the junction point between the first and second switching elements; an output detector means for detecting the output at the output terminal; two comparators for comparing the output of the output detector with reference signals at mutually opposite polarities; and two controllers for generating PWM signals to be supplied to the first and second switching elements, according to the outputs of the two comparators; PA1 (8) A three-value power supply device as described above, in which first and second switching elements are associated with a constant-current driving circuit for constant-current driving of the switching elements, and a saturation blocking circuit for preventing that the on-voltage of the switching elements becomes lower than a predetermined level; PA1 (9) A three-value power supply device having a high-voltage generator of a positive output; a high-voltage generator of a negative output; a positive-side switch for on/off supply control of the output of the high-voltage generator of positive output to an output terminal; a negative-side switch for on/off supply control of the output of the high-voltage generator of negative output to the output terminal; and a timing control device for on/off control, at the timings of output level switching, of each of the high-voltage generator of positive output, the high-voltage generator of negative output, the positive-side switch and the negative-side switch; PA1 (10) A three-value power supply device as described in the foregoing paragraph, in which the timing control device is adapted, at the switching of the output at the output terminal from positive to negative value, to turn off the positive-side a predetermined amount of time before turning-on of the negative-side switch; PA1 (11) A three-value power supply device as described above in paragraph 9, in which the timing control device is adapted, at the switching of the output at the output terminal from positive to negative value, to turn on the high-voltage generator of negative output a predetermined amount of time before turning-off the high-voltage generator of positive output; PA1 (12) A three-value power supply device as described in paragraphs (10) or (11), in which the timing control device is adapted, also at the switching of the output at the output terminal from negative to positive value, to turn off the negative-side switch or to turn on the high-voltage generator of positive output, earlier by a predetermined time; and PA1 (13) An image forming apparatus in which the developing bias is obtained by superposing, with the output of a desired DC power source, the output of a three-value power supply device according to any of paragraph (1) to (12). PA1 (1) An output detection circuit is provided, and the charging of the load capacitance by the high-voltage switch is terminated upon detection that the output has reached the intermediate value; PA1 (2) Between the base and emitter of the high-voltage transistors employed in the first and second high-voltage switch circuits, there is added a switch for shortcircuiting the base and emitter with a low impedance, when the high-voltage transistor is shut off.